XII 



THE ART OF PARASITISM 



Preview. Who qualifies? • Some host-parasite relationships : The host- 

 parasite conflict ; effects of a parasitic life ; keeping the cycle going ■ The 

 complexity of parasitic relationships : External parasites ; temporary para- 

 sites, periodic parasites, pennanent parasites ; internal parasites ; parasites 

 requiring one host, parasites requiring two hosts, malaria, parasites requiring 

 more than two hosts • Suggested readings. 



PREVIEW 



According to the definition, a parasite is one that "lives on or 

 within, and at the expense of some other organism," and thus might 

 include forms from the smallest, such as filtrable viruses and bacteria, 

 to some of the largest species. As a matter of fact, parasitism is 

 well-nigh universal, for examples are found among nearly all groups 

 of plants and animals. 



In many instances there appears to be a remarkable balance 

 between the parasite and its host. A dead host is of little use to a 

 parasite since it implies a loss of free transportation as well as board 

 and lodging. Consequently the existence of a parasite must be a 

 compromise, for it must be able to secure enough nourishment to 

 maintain and reproduce itself and yet do this either without injuring 

 too much the vitality of its host, or actually reducing its own numbers. 

 As a result of this rather elaborate compromise parasites have become 

 so adapted that they usually destroy only small portions of the host 

 tissue which usually can be replaced by regeneration. 



Whenever a parasite reaches a final host, the problem of propaga- 

 tion arises. Most parasites produce large numbers of eggs, cysts, or 

 spores that are discharged with the waste products of the host. 

 Through the medium of food or drink, these reach the next host, which 

 is sometimes intermediate or secondary, the parasites thus becoming 

 dependent upon the food habits of more than one organism to main- 

 tain their cycles. 



Most animal parasites are essentially carnivorous in their feeding 



habits. True carnivores, however, destroy their prey, whereas 



parasites as a rule do not, and while carnivores are much larger than 



their prey, parasites are smaller. Elton says, to summarize, "The 



H. w. H.— 15 21. "S 



